Optomechanics allows the transduction of weak forces to optical fields, with many efforts approaching the standard quantum limit. We consider force-sensing using a mirror-in-the-middle setup and use two coupled cavity modes originated from normal mode splitting for separating pump and probe fields. We find that this two-mode model can be reduced to an effective single-mode model, if we drive the pump mode strongly and detect the signal from the weak probe mode. The optimal force detection sensitivity at zero frequency (DC) is calculated and we show that one can beat the standard quantum limit by driving the cavity close to instability. The best sensitivity achievable is limited by mechanical thermal noise and by optical losses. We also find that the bandwidth where optimal sensitivity is maintained is proportional to the cavity damping in the resolved sideband regime. Finally, the squeezing spectrum of the output signal is calculated, and it shows almost perfect squeezing at DC is possible by using a high quality factor and low thermal phonon-number mechanical oscillator.Dramatic progress in coupling mechanics to light [1][2][3][4] suggests that such devices may be used in a wide variety of settings to explore quantum effects in macroscopic systems. Furthermore, such systems can be exquisitely sensitive to small perturbations, such as forces induced either by acceleration as in accelerometer [5] or by, e.g., coupling to surfaces or fields as in atomic force microscopy [6]. For such force measurements, a high quality factor (Q) mechanical oscillator acts as a test mass, transducing a force into a time-dependent displacement of the oscillator [7,8]. By using interferometric techniques to monitor the position of the oscillator, one can infer the force via optical signals. However, the radiation pressure coupling between the mechanical mode and optical mode has three consequences: photon shot noise, quantum backaction and dynamical backaction [1,9]. The dynamical backaction modifies the oscillator dynamics [10] and makes laser cooling [11,12] or amplification of phonons [13] in the mechanical system possible. Photon shot noise and quantum backaction, the former decreases with increasing input laser power while the latter increases with increasing input laser power, introduce two sources of noise on the displacement readout of the oscillator motion. An optimal compromise between these two noise sources leads to the standard quantum limit (SQL) in force sensing [7].The SQL, however, is itself not a fundamental limit. By using squeezed states of light [14], employing quantum nondemolition (QND) measurement [15], or by cavity detuning [16], the SQL can be surpassed. Here we show that in a coupled two-mode optomechanical system, if we drive it appropriately, the interaction between cavity photons and the mechanical oscillator will generate squeezed states of the output light. Measuring an appropriate quadrature of the output light field, we would get fewer fluctuations than that of the vacuum state, which makes it possib...

The transport of sound and heat, in the form of phonons, can be limited by disorder-induced scattering. In electronic and optical settings the introduction of chiral transport, in which carrier propagation exhibits parity asymmetry, can remove elastic backscattering and provides robustness against disorder. However, suppression of disorder-induced scattering has never been demonstrated in non-topological phononic systems. Here we experimentally demonstrate a path for achieving robust phonon transport in the presence of material disorder, by explicitly inducing chirality through parity-selective optomechanical coupling. We show that asymmetric optical pumping of a symmetric resonator enables a dramatic chiral cooling of clockwise and counterclockwise phonons, while simultaneously suppressing the hidden action of disorder. Surprisingly, this passive mechanism is also accompanied by a chiral reduction in heat load leading to optical cooling of the mechanics without added damping, an effect that has no optical analog. This technique can potentially improve upon the fundamental thermal limits of resonant mechanical sensors, which cannot be attained through sideband cooling.

Optomechanical systems provide a unique platform for observing quantum behavior of macroscopic objects. However, efforts towards realizing nonlinear behavior at the single photon level have been inhibited by the small size of the radiation pressure interaction. Here we show that it is not necessary to reach the single-photon strong-coupling regime in order to realize significant optomechanical nonlinearities. Instead, nonlinearities at the few quanta level can be achieved, even with weak-coupling, in a two-mode optomechanical system driven near instability. In this limit, we establish a new figure of merit for realizing strong nonlinearity which scales with the single-photon optomechanical coupling and the sideband resolution of the mechanical mode with respect to the cavity linewidth. We find that current devices based on optomechanical crystals, thought to be in the weak-coupling regime, can still achieve strong quantum nonlinearity; enabling deterministic interactions between single photons.PACS numbers: 42.50. Wk, 07.10.Cm, 42.50.Lc, 42.50.Dv Recent years have seen dramatic progress in realizing deterministic interactions between single photons, which has profound implications for future optical technologies [1][2][3][4]. The most striking success has been achieved with cavity quantum electrodynamics (cQED) [5][6][7][8][9][10][11][12], where photons inherent the saturation of a single two-level atom due to strong interactions between the atom and the cavity field. Alternative approaches have been explored based on slow-light-enhanced Kerr nonlinearites [13][14][15], single dye-molecules [16], strong photon interactions mediated by Rydberg atoms [17][18][19][20], enhanced nonlinearities in plasmonic systems [21,22] and atoms coupled to wave guides [23][24][25][26].Optomechanical systems, where light and mechanical motion are coupled by radiation pressure [27][28][29][30][31][32][33], are a promising approach to realizing strong photon interactions. Unfortunately no experiment has yet managed to reach the single-photon strong coupling regime. Recently it was noted that, in the weak coupling regime, there are still signatures of optomechanical nonlinearity [34][35][36]; however, strong coupling is required to achieve significant nonlinear quantum effects and deterministic photon interactions with optomechanics [37][38][39][40].In this Letter, we show it is not necessary to reach the quantum strong coupling regime in order to obtain large single-photon nonlinearities. Instead, in two-mode optomechanical systems with strong side-band resolution, the nonlinearity can be enhanced to the single-photon level by driving the system near an instability. In particular, as the strength of the driving field increases, the frequency of one of the optomechanical normal modes approaches zero and the associated harmonic oscillator length becomes large [41]. The increased quantum fluctuations associated with this mode result in an enhanced nonlinear interaction. We show that when the mechanical mode is sideband resolved with...